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Pork quality attributes associated with bilateral carcass variation¹

C. A. Stahl^*, H. Heymann, K. Adhikari and E. P. Berg^*,2

^* Department of Animal Sciences, University of Missouri, Columbia 65211; and Department of Viticulture and Enology, University of California, Davis 95616; and and Department of Human Nutrition, Kansas State University, Manhattan 66506

	Abstract

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The objective of this study was to identify bilateral differences in pork carcass quality as influenced by single leg shackling and suspension of electrically stunned pigs. Fifty-six Genepacker x EB market barrows (118 kg) were rendered unconscious and insensible to pain via electrical stimulation. Pigs were then shackled and suspended by the right hind leg before exsanguination and remained suspended until scalding. Initial (45-min) pH values of the LM at the 10th to 11th rib interface were different (P = 0.016); shackled side loins possessed a greater i.m. pH than those loins associated with the free side of the carcass (6.26 vs. 6.10). At 24 h postmortem, loins from the shackled side had lower (P < 0.001) L* values than loins from the free side in both the sirloin (53.08 vs. 57.61) and blade (54.61 vs. 57.09) regions. Additionally, b* values were greater (P < 0.001) for loins from the shackled side than loins from the free side in the center (13.81 vs. 13.01) and blade (14.72 vs. 13.68) regions, and in the blade region, a* values were greater (P < 0.001) in the LM from the shackled side than loins from the free side (7.37 vs. 6.26). Moreover, LM chops of loins from the shackled side that were aged 21 d were rated less (P < 0.05) tough (2.74 vs. 3.95) and more (P < 0.05) juicy (4.24 vs. 3.19) than chops from the free-hanging side. However, these differences were not evident after 41 d aging. From the results of this study, it can be concluded that bilateral differences exist within the LM of a pork carcass and that these differences are likely associated with the single-leg shackling and suspension of electrically stunned pigs.

Key Words: palatability • pork • quality • shackling • variation

	INTRODUCTION

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Consumers desire attractive and economically priced meat products that are nutritious and healthy, tender, juicy, flavorful, and contain no fat or additives (Jeremiah, 1998). Additionally, consumers assess meat products for a consistent size, shape, color, fat-to-lean and lean-to-bone ratio, texture, and cost per serving (McGill, 1981). As a result, product consistency has become the key to ensuring consumer acceptance of fresh pork (Roper, 2000).

Currently, the appearance of fresh pork varies both across and within packages found at the retail level (Tan et al., 2000). From a marketing perspective, this variation may cost the US pork industry more than $5.3 billion annually (van Laack et al., 1995). Market research has shown that the variability of pork meat can be attributed to genetics, breeding, nutrition, growth promotants, preslaughter handling and transport, immobilization, dehairing, and postslaughter handling (Cannon et al., 1995), but these factors alone are unable to explain the divergence in quality currently found within a single carcass.

Fischer and Augustini (1981) suggest that bilateral differences in ham quality can exist within a given carcass. Although research on this issue is limited, the current experiment proposed that similar bilateral variation can also exist within the loin. Consequently, the objective of this study was to determine if bilateral differences in fresh pork quality exist within the loin of a pork carcass as the result of single-leg shackling and suspension.

	MATERIALS AND METHODS

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Pig Slaughter
Fifty-six Genepacker x EB market barrows (118 kg) were delivered to the University of Missouri-Columbia abattoir for humane slaughter under USDA, Food Safety and Inspection Service inspection. Upon arrival, pigs were unloaded (no use of electric prods), inspected, moved into the stunning chute, rinsed with cool water, and electrically stunned (head only; 480 V). The right hind leg was shackled, and the pig was suspended by an overhead hoist immediately after stunning and just before exsanguination. Carcasses remained suspended by a single hind leg for the duration of exsanguination before scalding (approximately 5 min). After the scalding and dehairing processes, carcasses were gambrelled, eviscerated, washed, and inspected. All carcasses reached the cooler within 30 min. The loins from the shackled and free sides of each carcass were identified, HCW were recorded, and carcasses were placed in a still air cooler (4°C) for approximately 24 h. At 45 min and 24 h postmortem, i.m. pH was measured in the LM at the 10th to 11th rib interface of both the shackled and free sides of the carcass with a glass-tipped pH Star-Probe (SFK Technology, Peosta, IA) calibrated in pH 4.60 and 7.00 buffer solutions.

Carcass Fabrication and Quality Data Collection
Rough-cut, bone-in loins from both the shackled and free sides of the carcasses (n = 112) were removed from the belly perpendicular to the outside skin surface, extending in a straight line from a point on the most anterior rib immediately adjacent to the ventral edge of the psoas major. After the removal of the psoas major, loins were split at the 10th to 11th rib interface, de-boned, and separated into both anterior (3rd through 10th ribs) and posterior (10th rib to sirloin) sections. Each section of the loin muscle was trimmed of external fat (0.32 cm remaining), weighed (TR Series Toploading Balance; Denver Instrument Co., Denver, CO), and tagged. Boneless loin sections were allowed to bloom for 15 min, a duration shown to be sufficient for the stabilization of International Commission on Illumination L* (lightness), a* (redness), and b* (yellowness) values (Brewer et al., 2000). Then, the cut lean surface of the blade, center (10th rib of the anterior portion), and sirloin sections of each loin were used to obtain light reflectance measurements (L*, a*, and b*), using a Miniscan XE Plus spectrophotometer (Hunter Associates Laboratory, Inc., Reston, VA) standardized to a black and white tile. Objective measurements were recorded using a light source of D65 and a 10° standard observer.

Storage
Anterior and posterior sections of the boneless loin were weighed, vacuum-packaged, and randomly allotted to 21 or 41 d of storage within a commercial still air cooler (1°C). Storage durations of 21 and 41 d were selected to simulate the delivery of both domestic and export designated product, respectively. After storage, loin sections were removed from the vacuum-packaging, blotted dry, and reweighed for determination of whole muscle purge. Each exposed surface of the loin muscle (blade, center, and sirloin) was refaced by removing a single 1.27-cm-thick chop. Next, a 2.54-cm-thick chop was removed from each of the 3 freshly cut surfaces and provided 15 min to bloom. The proximal (bone) side of each chop was measured for i.m. pH, whereas the cut lean surface of the same chop was measured for color (L*, a*, b* values). Hue angle (arc-tangent [{b*/a*} x 360°]/[2 x 3.14]) and chroma (square root [a*]²+[b*]²) were calculated (Minolta, 1994).

Sensory Evaluation
Loins from the shackled and free side and that were aged 21 or 41 d (n = 12 loins from each side/aging period) were selected at random for sensory evaluation. Boneless, 2.54-cm-thick loin chops from each side of the carcass (posterior section) were placed on a Farberware Open Hearth Grill (Model 455N; Walter Kidde, Bronx, NY) and cooked to an internal temperature of 35°C, turned, and cooked to an ultimate internal temperature of 70°C. Temperature was monitored using copper-constantan wires (Omega Engineering Inc., Stamford, CT) and a 12-channel scanning thermocouple (Cole-Palmer Instrument Co., Vernon Hills, IL). Exterior edges of each loin chop were removed, and the remaining portion was cut into 1.27 cm³ cubes. Panelists were served 3 cubes of each sample on a toothpick in preheated 50-mL beakers (160°C) labeled with a 3-digit random number. Sample temperature was maintained with a metal pan filled with heated sand.

A trained descriptive attribute panel composed of 9 panelists evaluated the samples for tenderness and juiciness. A 10-cm line-scale anchored at both ends was used to measure the responses of the panel. The anchors were extremely tender to extremely tough for the tenderness attribute and extremely dry to extremely juicy for the juiciness attribute. The randomized serving order was established by using the experimental design feature of Compusense (Compusense five Release 4.4; Compusense Inc., Guelph, Ontario, Canada). Computerized ballots (Compusense five Release 4.4) were used for data collection. Three samples were presented to panelists per session, and 2 sessions were conducted per day. It took 8 sessions (4 days) to complete evaluations for each aging period. The panelists were given unsalted crackers (Premium; Kraft Foods, Northfield, IL) and Culligan water (Culligan Distribution, Columbia, MO) for cleansing their palates between samples.

Statistical Analysis
All loin quality data was analyzed as a complete randomized design using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC); individual carcass served as the experimental unit. Least squares means were calculated for i.m. pH, CIE L*, a*, b* values, color saturation, hue angle, purge loss, toughness, and juiciness from a model including carcass side and storage duration. Least squares means were separated statistically using Fisher’s LSD test when the F-test for treatment effects was significant (P 0.05).

A split-plot design in time was used for statistical analysis of the sensory data. The whole plot was loin side, and the subplot was aging period. The error term for the whole plot was loin side x panelist. Least square means were separated statistically using Fisher’s least square difference test when the F-test for treatment effects were significant (P < 0.05). All analyses were performed using procedures for the general linear model (PROC GLM) of SAS (Version 8e; SAS Inst.).

	RESULTS

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Intramuscular pH
At 45 min postmortem, loins from the shackled side had a greater (P = 0.017) i.m. pH than loins from the free side (Table 1). However, at 24 h, 21 d, and 41 d postmortem, no (P 0.241) differences in pH were observed between the shackled and free sides.

View larger version (33K): [in this window] [in a new window]   Figure 1. Interactive effects of bilateral symmetry and days postmortem on lightness (L*) values measured on the lean cut surface associated with the 3rd (blade section; P = 0.026) and 10th (center section; P = 0.897) ribs, as well as the sirloin (sirloin section; P = 0.002) portions of pork loins. Within a loin section, bars lacking a common letter differ, P < 0.001.

View larger version (38K): [in this window] [in a new window]   Figure 2. Interactive effects of bilateral symmetry and days postmortem on redness (a*) values (P = 0.003), yellowness (b*) values (P = 0.007), color saturation (P = 0.001), and hue angles (P = 0.088) measured on the cut surface associated with the third rib (blade section) of pork loins. Bars lacking a common letter differ, P = 0.003.

Whole Muscle Purge Loss
No differences (P 0.255) in purge loss were noted when comparing the anterior sections of the loin removed after 21 and 41 d of vacuum-packaged storage (Table 3). Even though rear leg shackling did not (P = 0.249) affect 21-d purge loss of the posterior section of the loin, shackling caused an increase (P < 0.05) in muscle purge from the posterior section of loins stored 41 d.

	DISCUSSION

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Intramuscular pH
Given that there is a strong interrelationship between lactic acid accumulation and the lean quality of fresh pork, previous research has focused upon understanding the regulation of postmortem pH decline. Although researchers have made great strides in understanding the numerous metabolic mechanisms involved in postmortem pH decline, they have yet to develop a universal protocol for its acquisition. As a consequence, many researchers obtain i.m. pH measurements from a single side of the carcass or in differing locations, assuming that the rate and extent of postmortem pH decline is comparable both within and between each side of the carcass. However, the empirical data collected in this study and by Fischer and Augustini (1981) suggest that this assumption may not apply in electrically stunned pigs; the extent of postmortem biochemical change differs in contralateral muscles when shackled and suspended by a single hind limb during the period between sticking and scalding. Research conducted by Fischer and Augustini (1981) demonstrated that the semimembranosus of the support hind leg was, on average, 0.19 pH units lower than the corresponding muscle in the free-hanging leg 45 min postmortem. Contrary to the ham, results of the present study suggest that postmortem pH is lower (0.16 pH units) in the free side than loins from the shackled side when measured at 45 min postmortem. The bilateral difference in postmortem LM pH is interesting, considering that the measurement of i.m. pH at 45 min poststunning is regarded as sorting criteria for muscle quality (van der Wal et al., 1995).

Measuring muscle pH accurately, consistently, and rapidly is difficult at the line speeds of commercial abattoirs. Consequently, it is more common for packing plants to measure pH at 20 to 24 h postmortem when the carcasses are hanging stationary in the chiller (Berg, 2000), a technique that has been shown to be a less accurate predictor of meat quality (Bowker et al., 1999). However, any bilateral differences in postmortem LM pH subsided by 24 h postmortem (Table 1), suggesting that both the industrial and academic fields should consider the development of a standard protocol regarding bilateral acquisition of i.m. pH at 45 min postmortem.

Objective Color
Bilateral differences existed in the LM L* and a* values of the sirloin and blade sections when obtained 24 h postmortem, but no differences were noted in the center section of the loin (Table 2). More importantly, greater L* values of sirloin and blade sections from loins from the free side corresponded to lower 45-min pH values compared with loins from the shackled side. This is consistent with the generally recognized dogma that 45-min pH is an excellent predictor of pork quality, especially color and water-holding capacity. However, Norman et al. (2003) observed the presence of both an ultimate pH and color gradient throughout the length of the LM. Moreover, it is unclear why the 45-min pH value measured in the center section (10th to 11th rib interface) was related to color of the blade and sirloin sections, but not the center of the loin. Muscle is quite extensible during the period immediately after exsanguination (Hedrick et al., 1994). Although a number of ante- and postmortem factors can influence pork color (Cannon 1995), the bilateral color differences observed in the present study may have resulted from differing degrees of tension placed upon muscle during the onset of rigor. Thus, it is plausible that prerigor shackling and suspension may have elongated or stretched the LM sarcomeres, resulting in more extensive rigor at the loin’s most anterior and posterior sections (blade and sirloin). This would cause the blade and sirloin sections to undergo a more extensive rigor bond, ultimately leading to the shrinkage of the filament lattice and transfer of water with the water-soluble pigment (myoglobin) out of the myofibril (Schäfer et al., 2000). Moreover, Schäfer et al. (2000) documented that sarcomere shortening was associated with a paler LM color. In a review by Kauffman et al. (1993), the extent of rigor mortis (as measured by a rigorometer) and evaluation of subjective thoracic limb rigidity could not segregate PSE, DFD, and red, soft, and exudative pork, but red, soft, and exudative, PSE, and DFD pork had greater sarcomere contraction than red, firm, normal pork.

Whole Muscle Purge
Lean pork varies tremendously in its ability to retain the natural moisture associated with muscle tissue (Forrest et al., 2000). Consequently, muscle purge, or drip loss, is a concern of the pork industry, especially the relationship between water-holding capacity and the subsequent losses incurred during processing and cooking (Huff-Lonergan, 2002). As just mentioned, drip loss from fresh pork is a result of lateral myofibrillar shrinkage (reduced spacing between actin and myosin) and the subsequent expression of fluids from the meat (van Heugten, 2001). Fluids have been shown to accumulate within the extracellular spaces over the first 24 to 48 h postmortem (Offer and Cousins, 1992). Previous research has shown that 24-h drip loss was not affected by sarcomere shortening, but drip formation after 7 days of aging at 4°C decreased linearly as sarcomere length increased (Honikel et al., 1986).

Even though purge losses from anterior loin sections did not differ between free and shackled sides after 21 or 41 d of storage, purge from posterior loin sections of the free sides was greater than that of the shackled side after 41 d of storage. Offer and Knight (1988) demonstrated that when the thick and thin filaments become more closely associated (onset of rigor), an increase in the expulsion of water occurs. It is possible, therefore, that the extensive rigor bond of the shackled side blade and sirloin sections caused an increase in the initial (<24 h) purge loss, thereby decreasing the amount of bound water within the loin before vacuum-packaging. Yet, it is more plausible that the differences in initial (45-min) pH between free and shackled sides were responsible for the observed differences in the water-holding capacity of these loin sections. Bowker et al. (1999) concluded that the rate of postmortem i.m. pH decline had a more pronounced effect on pork color and purge loss than any single time measurement of muscle pH. In the present study, the i.m. pH of the free side loin was lower than the shackled side at 45 min postmortem, and it is conceivable that the rate of pH decline within the free-sided loin may have been more rapid and contributed to the increase in whole muscle purge.

Sensory Evaluation
Tenderness is arguably the most important component of eating quality (Tarrant, 1992). In the present study, a trained sensory panel ranked 21-d aged LM chops from the shackled-side loins as being more tender and juicy than those from free-side loins (Table 3).

The degree of tenderness and/or toughness within a given muscle is partially determined by the postrigor contractile state (Hedrick et al., 1994), a condition caused by the amount of tension placed upon a muscle during the onset of rigor. Research has shown that increasing prerigor muscle length and tension until completion of rigor mortis resulted in longer sarcomeres and improved cooked meat tenderness (Herring et al., 1965) and increased tenderness. Recent findings (Wheeler et al., 2000) involving the variation in tenderness among unaged pork muscles suggested that there is a strong, positive correlation between sarcomere length and tenderness. Although sarcomere length was not measured in the present study, it is reasonable to assume that hind leg shackling and suspension for even a short period of time may have increased the tension sufficiently on the entire loin and resulted in the improvements in perceived tenderness observed in this study. This would have allowed the center portion of the loin muscle to stretch, potentially increasing its sarcomere length and decreasing its perceived toughness.

Meat juiciness is a palatability attribute that is important in conveying overall impressions of palatability to consumers (Hedrick et al., 1994). Consequently, meat juiciness has recently garnered a considerable degree of interest within the pork marketing chain, specifically as it pertains to both poor water-holding capacity and the overcooking of fresh pork products (Miller, 1998). Research has shown that the extent of sarcomere shortening (prerigor) contributes to the loss of water-holding capacity during postrigor storage (Honikel et al., 1986) and a subsequent decrease in product juiciness. In addition, Wheeler et al. (2000) found a moderately high correlation between LM sarcomere length and cooked pork juiciness. Holding true to our hypothesis of greater LM stretching, chops originating from loins from the shackled side were juicier than those chops coming from the free side of the carcass, which is very similar to the results of Wheeler et al. (2000).

	IMPLICATIONS

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Results from this study indicate that bilateral differences exist between loins of a pork carcass depending on whether they were from the shackled and suspended side or the free side of the carcass. More importantly, the common practice of single-leg shackling and suspension may influence initial (45 minute) intramuscular pH, objective color, purge losses of fresh pork, as well as cooked pork palatability of electrically stunned swine. These findings suggest that postmortem pH should be measured within 1 hour in both sides of the carcass for a more accurate prediction of total carcass quality and that each side of a pork carcass should be evaluated and sorted on an individual basis.

	Footnotes

 
¹ The authors express gratitude to M. Linville, D. Kemp, and D. Rhoads for technical assistance and data collection. We further acknowledge the efforts of M. Swaney-Stueve for the procedural support throughout the descriptive analysis section of this study.

² Corresponding author: BergEP{at}missouri.edu

Received for publication September 4, 2003. Accepted for publication October 28, 2005.

	LITERATURE CITED

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